CRUSHING AND GRINDING
• Dry Limestone-Dry Crushing i Iron Ore-Dry Cmshing_J____.
* Quartz-Dry Crushing 1 . .Argillaceous Limestone-Dry_
& Se.mi-Wet Crushing
The speed of course, is a very important item and ordinarily this should not exceed 900 ft. per minute, although there are rolls in successful operation at a speed of 1,000 ft. The difficulty is that with higher speeds, there is brought into play a grinding effect, due to slip of the rock particles on the roll shells, and this tends to abrade the shells and wear them out very rapidly. The best results are obtained by the use of moderate speed not exceeding 900 ft. per minute and with careful attention to the angle of nip, which has already been explained. The cylinder diameter of the rolls should always be such that the angle of nip will be sharp enough to grasp the piece presented and nip it without permitting slipping.
One of the great difficulties with rolls has always been the impossibility of feeding them equally throughout the entire width of the roll surface. The result is that grooves are worn in the roll shells in certain places, which seriously diminish their efficiency and result in the production of a material that is not properly sized. Many devices have been tried to obviate this difficulty. Often one of the rolls is flanged, the opposite roll fitting into this flange, the idea being to permit definite distribution of the feed over the entire roll face without spilling any of it. This variation has not been altogether successful. A second method has been to provide one of the rolls with a longitudinal movement along its axis; this movement is known as "fleeting." Usually the fleeting motion is small, the idea being to present a new surface continuously at any point where the rock may be delivered, thus avoiding
5"- 0.01 0.02 0.03 0.04
« p= HP PER CUBIC FOOT PER HOUR
FIG. 6.—Power consumed in rolls.
Maxi'mum Sfie of Parfides in Feed
2.2 2.4 ?.G ?.8 3.0
0 0.05 01 0.5 0.2 0.25 0.3 035 Q.4- 0.45 0-5 0.55 0.6 0.65 0.7 0.15 Max. Size of Particles in Finished Product.Reduction4tol FIG. 7.—-Size of feed for rolls.
doing all the work in one spot. Fleeting devices may be continuous and mechanical, or may be adjustable by hand. The fleeting device has been successful in a number